An aqueous solution of sodium chlorate and sodium chloride is conventionally produced by the electrolysis of aqueous sodium chloride in diaphragmless electrolytic cells. The extent of electrolysis is controlled to produce an effluent from the cell in which the sodium chlorate and sodium chloride have the desired ratio, usually in the range of about 1:1 to about 20:1 and preferably in the range of about 2:1 to about 15:1. The aqueous solution may be further processed to crystallize out the sodium chlorate for a variety of purposes, for example, in the production of chlorine dioxide for use in the bleaching of chemical cellulosic pulps, by reduction in the presence of a strong mineral acid, usually sulphuric acid, or the aqueous solution may be used as such, for example, in the production of chlorine dioxide.
In the electrolysis of sodium chloride to form sodium chlorate, hexavalent chromium conventionally is added to the brine feed to the cell to improve significantly the current efficiency of the cell in the conversion of sodium chloride to sodium chlorate, by suppressing the reduction of hypochlorite at the cathode. Usually, sodium dichromate is employed to provide the hexavalent chromium.
The aqueous sodium chloride electrolysis produces a gaseous by-product, mainly hydrogen but also some oxygen, chlorine and steam, especially when metal anodes are employed and the electrolysis occurs at elevated temperature. The by-product gas stream is passed through a condenser, wherein the steam is condensed to form an aqueous solution of hypochlorous acid, typically about 2 to 15 gpl HOCl, which also contains small amounts of dissolved chlorine. The highly corrosive nature of the condensate restricts the methods of dealing with it. For example, use in the formation of make-up liquor for chlorate production introduces severe corrosion problems. In addition, if the condensate were to be reintroduced directly to the cells, water imbalances could result, which are undesirable.
Hypochlorite also usually is present in the sodium chlorate product stream from the cells as a result of inefficient chlorate formation and is removed by so-called "dehypoing", either prior to crystallization of the sodium chlorate where the sodium chlorate is desired in that form, or prior to discharge of an aqueous solution of sodium chlorate and sodium chloride where the sodium chlorate is desired in that form. Such dehypoing is conventionally effected by heating the sodium chlorate solution at an elevated temperature for a time sufficient to remove most of the hypochlorite by conversion to chlorate and then by treatment with a reducing chemical to remove the remainder.